Police around the world are still searching for the Lipinski Stradivarius violin, stolen from Milwaukee Symphony concertmaster Frank Almond on Monday night.

Almond had just finished a concert and was taking the instrument back to his car when he was assaulted.

The violin is worth millions of dollars and is a one-of-a-kind instrument, and so within hours, the Milwaukee police enlisted the help of INTERPOL and the FBI’s art crimes unit.

That unit was founded a decade ago by experienced investigator Bob Wittman. He says investigating art crimes is different from other kinds of thefts.

"It doesn't matter if it's a Chevrolet or a Monet when you first start," he says. "But once you get into the secondary part of that investigation, then you start having to look at the collector markets, the individuals who are involved in the selling and buying of these types of things and having the business background in the art world."

Wittman has since retired from the agency and now lives in Pennsylvania and runs a private consulting firm in the same field. He is also the author of the memoir, Priceless: How I Went Undercover to Rescue the World’s Stolen Treasures.